House martin Delichon urbica, adult collecting mud, Kildonan, Isle of Arran

Getting ready to go

A bird's body tells it exactly when to migrate.

How a bird's body prepares for migration

Each year, at the same time, glands in its body release special chemicals – called hormones – into its system.

Natural signs, including the gradually changing day length (in the UK days get shorter in late summer and longer in early spring), cause this to happen automatically. These hormones make birds behave differently. They start flying around restlessly and begin to gather into flocks.

In spring, hormones also start preparing a bird for breeding. Its sexual organs, which have been hidden away during the winter, enlarge in preparation for mating. 

This change is another signal to leave for summer quarters. Some birds may pair up during the journey, but they wait until reaching their destination before looking for a place to nest.

It's time to start eating!

The hormones also trigger a feeding frenzy. The bird spends every minute of daylight stuffing itself with food, stocking up for the long journey ahead. Its body has a special ability to turn food quickly into fat. 

The fat forms a layer beneath its skin, which is converted into energy as the bird flies. Smaller species can gain 3-4 per cent of their body weight a day during this time; a sedge warbler almost doubles its weight from 10 g to 20 g in just three weeks. This extra load gives it enough energy to fly all the way to Africa.

Once the migrants are loaded with food and ready to go, they wait for the weather to fire the starting gun. A spell of calm weather with clear skies is usually the signal. This weather is caused by high atmospheric pressure (an anticyclone), and is typical for the UK during late summer and autumn. 

Low atmospheric pressure (a depression) causes wind, cloud and rain. This is bad for migration, so birds sit tight until it has passed.

Many birds start their southerly migration in short feeding hops, moving to reedbeds and other favoured areas on the south coast. As they fatten up, bird ringers record the amount of fat birds are carrying before their migration. 

Depending on the summer, birds can find plenty or very little food at this time, so the ringers' activity helps to show how healthy the countryside is for birds.

Perfect timing

The secret of successful migration is timing. If birds leave too early, they may run out of energy before finishing their journey. If they leave too late, they may run into bad weather along the way.

Each species has its own schedule. Adult cuckoos are off by August. Their parental responsibilities ended as soon as they laid their egg in another bird’s nest, so they have no reason to stay. House martins may hang around until October. They have to wait until their second – or even their third – brood has fledged, before the whole family heads south together. 

Leaving in waves

Birds of one species do not all migrate together. With many wading birds, the females set off first, leaving the males to stay behind and raise the chicks. The males leave as soon as the chicks can fend for themselves. Dunlins from Greenland reach the UK in three waves: first the females in July, then the males in August, and finally the young in September.

Cuckoo Cuculus canorus, perched in the top of a tree, Co. Durham